The term "foam", as used herein, designates a mixture of liquid, gas, and a surfactant that gives the liquid a film strength which permits the formation of long lasting bubbles when the mixture is agitated to convert it into a mass of bubbles. The liquid used is normally water, and the gas is usually air, because these ingredients are of low cost; but other gas and/or liquid can be used when compatible with the surfactant.
Various compounds are used as surfactants, and these can be purchased on the open market. Some of these compounds are proprietary. The strength of the film depends upon the characteristics of the surfactant, and the amount of the surfactant in the liquid-gas mixture, as will be more fully explained.
In order to have small particulate matter efficiently trapped, it is necessary for the particle to contact with a bubble of the foam and burst the bubble. As the bubble bursts, the gas in the bubble escapes; the bubble implodes; and the liquid film of which the bubble was made coats the particle. Small particles do not burst large bubbles and are not wetted or trapped by the foam. The smaller the bubble, the smaller the particles that can be trapped. This invention traps all detectable particles as a result of smaller bubbles made with higher strength foam.
The foam described in this specification has the bubbles burst by contact with small particles of material (dust) and the bursting bubble wets the particle. Particles as small as one micron are readily wetted. As this effect proceeds, the foam is destroyed by contact with the particles. The wetted particles must then be either
(1) brought together, PA1 (2) made to contact larger particles, or PA1 (3) brought into contact with a surface, which may be additional foam.
If the foam is injected into a free-falling aggregate (at a transfer point between belts, for example, or injected into a crusher along with the aggregate), the mechanical motion of the aggregate will provide the required particle-to-particle contact. When the foam is injected into an aggregate which is all fines (one to two hundred micron), some means must be provided to cause the wetted particles to coalesce. This is readily accomplished by use of a cyclone, as disclosed in the patent application Ser. No. 527,812, filed Nov. 27, 1974 referred to above, now U.S. Pat. No. 4,000,992, to which this application is related.
Another advantage of small bubble foam is that it can be ejected from nozzles at considerable pressure and resulting high velocity. This will be explained more fully in the description of the preferred embodiment.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.